A mediator variable is one of a set of "third" variables that are used in statistics to explain away the causal factors that are used to explain the results of research. It explains a relationship between the independent and dependent variables. For example, a study may show a positive correlation between exposure to media violence and committing crimes. So this research would suggest that watching violent media is a causal factor of committing crimes. But there are many more factors involved in what makes a person commit crimes.The researcher might inject information on poverty levels as a mediator variable as a partial explanation of why crimes occur.